Photopolymerizable flexographic elements for the manufacture of flexographic printing plates usually consist of a flexible substrate, an adhesive layer, a photopolymerizable layer, a release layer and also a protective film for the photopolymerizable layer, which is removed prior to exposure to light. The supports used in commercial flexographic elements are usually PET films.
Perfectly registered set-up of PET-supported flexographic printing plates on the impression cylinder is usually effected using double-sided adhesive tape. Usually the flexographic printing plates are mounted outside the printing press on the dismantled impression cylinder, for which purpose frequently special mounting equipment is used. The impression cylinder thus provided with the plates is then remounted in the printing press. If it is neessary to correct the position of the flexographic plate, the impression cylinder to which it is stuck must be again dismantled from the printing press and the flexographic plate unstuck. Then a new piece of double-sided adhesive tape is stuck to the impression cylinder, onto which the flexographic plate is restuck, and the impression cylinder is again remounted in the press. This, however, is a complicated and time-consuming procedure.
In the prior art, water-developable, photopolymerizable high-pressure plates are laminated to PET supports or metallic supports of aluminum or steel. To ensure good adhesion of the photo-polymerizable layer on metallic supports, used can be made, for example, of a combination of two adhesive layers comprising a prime coating and an intermediate layer. For examples of this usage reference is made to DE-A 3,045,516 or EP-A 333,012. Printing plates on thin metallic supports are much easier to mount on impression cylinders. Attainment of perfect registration is further alleviated by providing high-pressure plates with a thin support of magnetizable steel, in known manner, (Print-it No. 2, June 1999, BASF Drucksysteme GmbH, page 12-13). Such high-pressure plates can be simply and quickly mounted on magnetic impression cylinders, in perfect registration.
Flexographic printing plates on metallic supports are well known. Patents or patent applications relating to flexographic printing plates frequently mention, in the description, that metals, as a general class, are suitable for use as materials for the supports. Reference is made here, by way of example, to EP-A 992,849 (section [0030]) and EP-A 474,178 (page 3, lines 50-54).
However, the working examples of such patents or patent applications usually illustrate the use of flexographic elements supported by only polymer films, particularly PET films. Only EP-A 332,983 gives working examples of flexographic printing plates on metallic supports. However, the flexographic element disclosed in said reference is not an organically developable element based on thermoplastic elastomeric block copolymers but a water-developable element based on specific polyvinylalcohol derivatives, as binding agents. Such binding agents are taken from high-pressure plate technology, and such flexographic printing plates can only be used with UV inks. No organically developable flexographic printing plates on flexible metallic supports are as yet commercially available.
Hitherto only photopolymerizable flexographic elements on a double support of PET film and an aluminum sheet (eg Nylocoat® LA 116 (BASF), Cyrel® CLAM (Du Pont)) have been commercially available. These plates comprise a photopolymeric layer which is not directly attached to the aluminum support, but they are conventional flexographic printing plates supported on a PET film, which is stuck to an additional aluminum support by means of, say, double-sided adhesive tape. This requires an additional process step, which is very time-consuming and labor-intensive. There is also the risk that when the plate is being developed the washout agent may cause the adhesive tape to lift from the aluminum support. Moreover, the interlayer adhesion between the PET film and the aluminum sheet can break during the printing process due to mechanical stresses. Since the loss of adhesion can alter the register, it is an exceptionally undesirable effect.
The flexographic printing plates on double PET/aluminum supports described therein are mainly used for upgrading products of sheet-fed offset machines, for example by lacquering or gold printing (cf eg “Inlineveredelung ueber Flexolackierwerke”, published by Deutscher Drucker 29 (1999) w2 - w6). Flexographic printing plates used for this purpose are therefore also referred to as coating plates. In this field particular importance is attached to accuracy of register. Modern flexocoating machines in sheet-fed offset machines are frequently equipped with quick-action clamping bars or with fully automatic plate draw-in devices only suitable for drawing in printing plates having an aluminum support. Customers in offset printshops prefer flexographic printing plates on double PET/aluminum supports by reason of their distinctly higher dimensional stability as opposed to flexographic printing plates on film supports. In production printing, aluminum is distinctly more resilient to mechanical stresses than polyester. Even in the case of repeat orders involving repeated clamping and unclamping, the dimensional stability of the printing plates on aluminum supports is assured.
In view of the many advantages of flexographic printing plates on metallic supports, it would be desirable to apply the elastomeric photopolymerizable layer directly to the metallic support, in order to avoid the aforementioned complicated and elaborate manufacturing process.
In practice, however, the skilled person wishing to use metallic supports for flexographic printing plates faces a number of problems.
Photopolymerizable flexographic elements are usually prepared by melt extrusion. The photopolymeric composition is discharged through a slot die in between a protective film and a supporting PET film and the resulting composite is calendered. However, this technique cannot be simply transferred to supporting foils. Apart from the fact that it is industrially very exacting to carry out calendering on a metallic support, there is primarily the problem of corrugation. Due to their different coefficients of thermal expansion the metallic support and the protective film show different degrees of shrinkage when the melt-extruded photopolymeric composition is cooled. The difference in the shrinking behavior of the metallic support and the protective film leads to warping of the flexographic element, which is manifested by corrugation. Such flexographic elements cannot be used by reason of their high thickness tolerances.
The first step in the manufacture of flexographic printing plates of photopolymerizable flexographic elements is usually a so-called back exposure through the transparent supporting PET film. This serves inter alia to establish the relief depth and also to ensure that the individual halftone dots are firmly fixed to the substrate. The back exposure first of all causes a portion of the photopolymerizable layer to be completely polymerized, namely the lower region in contact with the supporting film. The upper portion of the layer remains unpolymerized at this stage. The upper portion is then converted to the actual printing relief in a second process step by front imaging. Thus the individual halftone dots of the flexographic printing plate do not adhere to the supporting film itself but are located on a layer of polymerized material in full-surface adhesive contact with the supporting film. The back exposure method is illustrated for example in “Technik des Flexodrucks”, pp 148 et seq, 4th Edition, 1999, Coating Verlag, St. Gallen, Switzerland. It is of course impossible to carry out any exposure through the back of a flexographic printing plate on a metallic support. Consequently, when the flexographic element is developed following front exposure through the negative the unpolymerized material of the unexposed regions is removed right down to the support. Thus there is the risk that particularly those individual halftone dots which have only a small base area and weak adhesion will be torn away from the support during development or during the printing operation.